According to U.S. Pat. No. 4,217,185, electrolytic reduction of tetrachloropicolinic acid ("tet-acid") in basic, aqueous solution at an activated silver cathode yields the 3,4,6- and 3,5,6-trichloropicolinic acids ("3,4,6-T" and "3,5,6-T") which in turn may be further reduced to 3,6-dichloropicolinic acid ("3,6-D"), a highly active herbicide. (The disclosure of the latter patent is incorporated herein by reference.)
In basic solution, it of course is the tetraand trichloropicolinate anions which actually are reduced (and 3,6-D is recovered as such by acidification of the reduction mixture). It is advantageous in several respects to be able to carry out a reduction of the disclosed type in the absence of organic solvents. However, the tet-acid is not very soluble in aqueous bases and the maximum attainable concentration of the corresponding picolinate anion is correspondingly low. The presence of any substantial amount of the undissolved tet-acid tends to result in particle aggregation and a type of foaming action (as a consequence of gas liberation at both electrodes). The latter difficulties can be largely avoided by adding the tet-acid incrementally during the initial stage of the reduction (together with an equivalent amount of base, dissolved in a minor portion of the reduction mixture). However, this does not have the effect of raising the concentration of the polychloropicolinate anions and the reduction rate is not increased.
It is apparent from the patent that either or both of the intermediate trichloro acids (if available other than by reduction of the tet-acid) could be considered as a starting material for 3,6-D production by the disclosed reduction process. However, it is not apparent that any increase in the production rate of 3,6-D would result. The inference may be drawn from the patent that the trichloro-acid is not significantly more base-soluble than the tet-acid. Having one less chlorine substituent than the tet-acid, the trichloro acids should be less chlorocarbon-like but would also be expected to be somewhat weaker acids; so it cannot be presumed on the basis of theoretical considerations that the tri-acids would be any more base-soluble.
It is also apparent that pre-formed mixtures of the tri- and tetrachloro acids--not necessarily in any ratio inherent in the disclosed stepwise reduction--could be employed as starting materials for 3,6-D production. The production of analagous acid--including 3,6-D itself, by acid hydrolysis of corresponding trichloromethylpyridines is known--as discussed below. Thus, if no hydrolysis of ring-chlorines occurred, acid hydrolysis of 3,4,6- and/or 3,5,6-trichloro-2-trichloromethylpyridine might be a feasible alternative route to the trichloro-acid intermediates. However, no substantial advantage over in-situ production of the latter acids from tet-acid is apparent. The amount of NaCl produced per pound of 3,6-D produced in the reduction process would be reduced but this could well be offset by the necessity of disposing of other by-products formed in the production of the trichloro-acids.
Furthermore, the prior art provides good reason for concern about the likelihood of ring-chlorine hydrolysis if production of the trichloro acids by hydrolysis is attempted. According to McBee et al, Ind. Eng. Chem., Vol. 39, page 389 (1947), the structures of 3,5-dichloro- and 3,4,5-trichloro-6-(trichloromethyl)pyridine were proven by "hydrolysis" to the corresponding trichloropicolinic acids. However, Seyfferth, Journal fur Praktische Chemie, Vol. 34, (1886) reported that hydrolysis of a "hexachloropicoline" (a trichloro, 2-trichloromethylpyridine, necessarily) gave a dichloro, hydroxypicolinic acid, rather than the trichloro acid; hydrolysis of a pentachloropicoline gave both the dichloro- and a hydroxy, monochloropicolinic acid.
There is also the consideration that the hexachloropicolines have only been available (by other than reduction) as components of not readily resolved mixtures of catalyst and by-products formed in the manufacture of symmetrical tetrachloropyridine ("sym-tet"). That is, U.S. Pat. No. 4,256,894--the disclosure of which is also incorporated herein by reference--discloses a process for production of symmetrical tetrachloropyridine ("sym-tet") by the chlorination/chlorinolysis of 2-chloro-6-(trichloromethyl)pyridine in the liquid state and in the presence of a Lewis acid catalyst, such as ferric chloride. Quite substantial amounts of other products form, even under optimum conditions, and must be separated from the sym-tet, as by distillation. Said other products include the 2,3- and 2,5-dichloro-6-(trichloromethyl)pyridines, the 2,3,4- and 2,3,5-trichloro-6-(trichloromethyl)pyridines, heptachloropicoline and pentachloropyridine. According to the patent, the reaction mixture is stripped of CCl.sub.4, HCl and Cl.sub.2 and then vacuum distilled to provide a first overhead cut comprising the sym-tet, the pentachloropyridine and some of the 2,3-dichloro-6-(trichloromethyl)pyridine and a second overhead cut comprising the rest of the latter compound and the 2,3,4- and 2,3,5-trichloro-6-(trichloromethyl)pyridines. The latter cut may be taken to include the heptachloropicoline (which is known to be hydrolyseable to tet-acid). However, FeCl.sub.3 can be expected to co-distil, even if the "hepta" is not included in the cut. FeCl.sub.3 is also quite soluble in polychloropicolines. Accordingly, FeCl.sub.3 separation would be expected to be difficult. This is of no great consequence to sym-tet production, but is a deterrent as to attempting to utilize sym-tet by-products as a starting material for 3,6-D production.
Thus, neither the desirability or feasibility of utilizing sym-tet by-products as starting materials for the production of 3,6-D is made apparent by the prior art.